Swirling-type furnace operating method and a swirling-type furnace

ABSTRACT

The invention relates to heat engineering and can be used for designing and reconstructing furnaces for industrial boilers. More advantageously the inventive furnace can be used for burning coarsely dispersed broken combustibles, coal and schist. The invective swirling-type furnace operating method consists in returning the controllable amount of a coal-ash mixture from an ash catcher located behind a combustion to the internal surface thereof. Said swirling-type furnace comprises a combustion chamber provided with a sloping bottom which is formed by slops of lower parts of the walls thereof, a bottom-blowing device arranged under the sloping bottom mouth and an inclined burner which is used for supplying a fuel-air mixture and is fixed to the combustion chamber wall. The furnace is provided with the ash catcher located behind the combustion chamber and a recirculation ash channel, whose one end communicates with the ash catcher and the other with the combustion chamber internal space. The output openings of the channel are positioned between the sloping bottom mouth and the fuel supply burner or/and in a secondary combustion area, provided with an additional burner which is used for supplying the coal-ash mixture from the ash channel to the combustion chamber and is arranged therein. The ash catcher is selected according to a condition of catching particles whose size is greater than 0.5 mm. The ash channel can be provided with fly ash transporting and sorbent supply means.

FIELD OF THE INVENTION

The invention relates to heat engineering, and more specifically, to furnaces for burning coarsely pulverized fuel, and can be best used for burning crushed coal fuel and shale.

BACKGROUND OF THE INVENTION

The basic parameters of industrial furnaces are their economical and ecological parameters, the former of which are mainly determined by the completeness of combustion and the costs of fuel preparation, and the latter mainly depend on the quality of flue gas that is released into the atmosphere.

From the viewpoint of uncrushed fuel combustion and the ecological parameters, fluidized bed furnaces show good results.

A furnace with a dense fluidized bed in its bottom part is currently in use (RU,A, 2094700).

Here is how this furnace operates:

Fuel is supplied to the ash recirculation channel and comes to the furnace with the dispersed substance on the boiling bed grate. Pressurized air that has been heated in the air heater is supplied under the boiling bed grate forming a boiling bed of the fuel and dispersed substance mixture. The air velocity in the furnace section is selected so as to pneumatically transport small particles of the dispersed substance and burning fuel particles to the furnace throat from where they go into a high-temperature cyclone. After separation in the cyclone, the solid phase of flue gas returns through a non-mechanic valve by the ash recirculation channel to the furnace to the area of the boiling bed border, and filtered flue gas is directed to the transition gas duct, the convective stack, the air heater, and then to the chimney. Because fuel particles that did not burn are returned to the boiling bed, fuel is burned quite completely.

A weak point of such furnaces is a low temperature drop due to low temperatures in the furnace chamber, which makes it impossible to produce high-temperature-high-pressure steam that is required by the economical specifications of the boiler plant.

A great part of existing furnaces have high-temperature combustion modes, but they are only possible if fuel is pulverized finely in advance. This results in high operation costs, a risk of slagging, and a higher content of nitrogen oxides in the flue gas.

There is a swirling-type low-emission furnace currently in use (RU, A, 2067724), which contains a combustion chamber with at least one downward-tilted burner installed on its wall to supply the fuel/air mixture, with one prism-shaped dry-bottom hopper with a slot-type mouth that is formed by the slopes of the walls of the combustion chamber bottom portion, and a bottom air inlet device under the dry-bottom hopper mouth. The burner is formed by at least two fuel/air mixture supply channels, which are located one above another. Each of the channels has a device for controlling the fuel/air ratio, and the said devices are so chosen that the ratio of the air amount to the fuel amount for the overlying channel is always greater than for the underlying channel. When this furnace is operating, a mixture of pulverized fuel and air is fed through both channels of the burner, and air is supplied through the bottom air inlet device. An excessive amount of oxygen is supplied into the top portion of the combustion chamber, whereas the concentration of fuel particles, which come from the overlying burner channel, is quite high in this area. This provides for a relatively high burning temperature with excess oxygen in this zone and consequently, an efficient fuel burnout. The charging of fuel into the middle portion of the furnace is preferably done from the underlying duct with a deficient amount of oxygen. As a result of the interaction between the fuel/air mixture flowing out of the duct and the air fed from the bottom air inlet device, a vortex zone is created whose major part is characterized by an oxygen deficiency and a relatively low maximum temperature; this zone serves as the reduction zone. Fuel that is supplied to each channel has a preset fractional composition, which can be provided for, e.g. by using a dust concentrator. In this case, fine fuel is supplied to the overlying channel, and it has enough time to burn near to that channel establishing a required temperature level, whereas the relatively coarse fuel is fed to the underlying channel, and it burns successfully in the vortex zone. Therefore, in the currently known furnace, there is multiple circulation of fuel particles in the low-temperature reduction zone, and at the same time fine particles that are carried away from the vortex zone are burned up in the high-temperature area, which is enriched with oxygen. Such a furnace operates successfully when pulverization systems are used, i.e. on condition that fuel is pulverized in advance using e.g. separator mills. The existing pulverization systems provide for pulverized fuel fineness (residue on an R90 sieve) of 40 to 60 percent for brown coal and shale and 15 to 40 percent for black coal. It is obvious that the production of such fine fuel requires substantial amounts of energy and special expensive machinery. Moreover, pulverized fuel is a dangerously explosive substance. In the event when in the operation of the existing furnace crushed lump fuel is fed to the burners (the maximum size of fuel pieces after crushing is usually 15 mathematical model, and 25 mm for high-moisture fuels), gravitation takes the fuel to the bottom part of the furnace, which leaves the top part of the furnace practically empty of fuel, and the temperature in this top part is not high enough to burn fuel particles that are carried away from the vortex zone. For a multiple circulation of large fuel particles and for the creation of a vortex zone, the velocity of the bottom air flow has to increase significantly. That results in poorer economical parameters and a dramatic increase of heat losses with mechanical underburn, as fuel particles are dried during circulation, and some of them break and are shot to the top part of the furnace by a powerful bottom air flow before they have had time to burn out. As the temperature in the top part of the furnace is lowered, these particles cool down and stop burning. In the standard most popular furnace design, steam superheaters are located in the top part of the furnace, and since due to the above the convective surfaces are underloaded, maintaining the nominal temperature of steam that is supplied to the turbine becomes a problem.

DISCLOSURE OF THE INVENTION

It is the object of the present invention to design an operation method for a swirling-type furnace, which would enable the swirling-type furnace to operate when non-pulverized coarse fuel is fed into the furnace chamber, and at the same time raise the completeness of fuel combustion and raise the heat load of the convective surfaces.

Another object of the present invention is to design a swirling-type furnace that would implement the method.

With the first object in view, operation method of the swirling-type furnace, which contains a combustion chamber, has a vortex zone and an afterburning zone, a bottom air inlet device, and a coal/ash mixture catcher for flue gases, which includes feeding the fuel/air mixture and the bottom air flow to the furnace, in accordance with the invention, the fuel/air mixture containing coarse crushed fuel is fed into the vortex zone of the chamber, and a regulated amount of the coal/ash mixture that is collected from flue gases is returned to the selected zone in the combustion chamber inner space.

The coal/ash mixture that the ash collector collects from flue gases is a mixture of ash and coke particles. The return of a regulated amount of the coal/ash into the combustion chamber increases the time that unburned coal particles from the coal/ash mixture spend in the furnace chamber, which raises the completeness of combustion significantly.

Fuel can be returned to the vortex zone. In this case, efficient afterburning is ensured due to multiple circulation of especially large fuel particles.

Fuel can be returned to the afterburning zone, which is most frequently located in the top part of the furnace chamber. In this case, the furnace chamber inner space is loaded efficiently with coal/ash mixture containing unburned particles that have, however, been already prepared well during the previous cycle. The return of unburned but heated, dried, and partly broken fuel particles to the afterburning zone increases the heat load of the convective surfaces.

With the second object in view, in addition to the basic design elements of all swirling-type furnaces (combustion chamber with a dry-bottom hopper formed by the slopes of the bottom parts of the combustion chamber walls, a bottom air inlet device under the dry-bottom hopper mouth, and a downward-tilted burner to supply the fuel/air mixture, which is installed on the combustion chamber wall), the swirling-type furnace where the method is implemented contains, in accordance with the invention, an ash collector behind the combustion chamber, and an ash circulation duct with a dust transportation device; one end of the ash circulation duct connects to the ash collector, and the other end connects to the combustion chamber inner space; the outlet of the channel is in the area of the selected zone of the inner space of the combustion chamber. A swirling-type furnace of such a design can work on almost unprepared fuel, i.e. no pulverization is required whatsoever, which reduces the furnace operation costs greatly. This is possible because due to the ash collector and the ash circulation duct unburned fuel particles are returned to the furnace, which can happen many times until all of the fuel is burned out. That raises the completeness of fuel combustion and increases the heat load.

It is preferred to install a coal/ash mixture consumption regulator in the ash duct.

It is preferred to install a sorbent input medium on the ash duct.

The ash circulation duct outlet can be located between the dry-bottom hopper mouth and the fuel feed burner. In this case, fuel is returned to the vortex zone of the furnace.

It would be preferred to install an additional burner on the combustion chamber wall in the afterburning zone. In this case, the ash circulation duct outlet can be combined with the said burner. If such a design is used, fuel is returned to the afterburning zone, which increases the charge of the heat transfer surfaces.

BRIEF DESCRIPTION OF THE DRAWING

The invention is illustrated with a drawing, which schematically illustrates the vortex furnace made in accordance with the invention and implementing the claimed method.

PREFERRED EMBODIMENT OF THE INVENTION

With reference to the drawing, the swirling-type furnace comprises a prismatic combustion chamber 1 with a dry-bottom ash hopper 2. Walls of the bottom part of the combustion chamber 1 create a dry-bottom hopper 2. A bottom air inlet device 4 equipped with a nozzle 5 is disposed beneath the mouth 3 of the dry-bottom hopper 2. On the wall of the chamber 1, there is a downward-tilted burner 6. Behind the combustion chamber 1 along the flue gas flow, there is the ash collector 7. The ash collector 7 can be implemented in any known way, e.g., as a cyclone or a blind design. Between the ash collector 7 and the combustion chamber 1, there is a branching ash duct 8, whose inlet 9 connects to the ash collector 7. Through outlets 10 a and 10 b, the ash duct connects to the inner space of the combustion chamber: through the opening 10 a with the vortex zone (marked as W on the picture) and through the additional burner 11 on the wall of the combustion chamber 1 with the afterburning zone (marked as P on the picture). The ash duct 8 is equipped with the coal/ash mixture supply regulator 12. The regulator 12 can be implemented in any known way, e.g., as a damper. Depending on the features of the fuel used, the coal/ash mixture can be returned to one of the zones (vortex or afterburning) or to both. The amount of the mixture and the direction are controlled using the abovementioned regulators 12 and 13. The location of the additional burner is chosen depending on the location of the afterburning area, preferably in its section where the temperature is the highest. If the invention is implemented as it is shown on FIG. 1, the additional burner 11 to supply the coal/ash mixture is located in the top part of the furnace chamber 1, because at such a design of the furnace chamber, the afterburning zone is located in the top part of the furnace. In other cases such as invert furnaces, the afterburning zone can also be placed in the bottom part of the furnace. If required, the ash duct can be equipped with a sorbent supply medium (not shown on the picture). The furnace operates as follows. After crushing, fuel (with no pulverization system used) is supplied to the combustion chamber 1 through burner 6. The size of fuel particles is only limited by the geometrical parameters of the burner 6 for the supply of the fuel/air mixture. Small particles burn in the parallel flow, whereas larger ones are directed to the bottom part of the combustion chamber with air. The interaction of the fuel-air flow from the burner 6 and the bottom air flow coming from the mouth 3 of the dry-bottom hopper 2, a vortex zone is established where large fuel particles are burned by means of multiple circulation. As fuel particles burn out and crack, they become smaller and lighter, their sailing capacity increases, the wandering speed reduces, and part of them is carried to the top part of the furnace without enough time to burn out. On the exit from the furnace chamber 1, flue gases reach the ash collector 7, which catches particles of ash and fuel that have not burned completely. The caught particles are stored in the ash collector 7 bunker and then taken to the selected zone through the ash duct 8, the outlets 10 a and 10 b, and the additional burner 11, to the afterburning zone through the burner 11 and to the vortex zone through the outlet 10 a. The amount of air and the velocity of the air flow that are required to complete burning the supplied coal/ash mixture and to prevent its escape by the bottom air flow are regulated in the usual manner. A regulated amount of the coal/ash mixture, which was caught by the ash collector 7, is returned to the selected zone.

The amount of the product that is returned into the selected zone depends on the parameters of the fuel (ash content, particle size, volatile content, etc.) and the product (content of unburned carbon and fractional composition). The more unburned carbon and the smaller the particle size in the product, the bigger the share of the product that is returned into the afterburning zone, and the bigger the unburned particles, the bigger the share of the product that is returned into the vortex zone.

If the ash content of the fuel is low, most of the coal/ash mixture that the ash collector catches is returned to the selected zone, and if the ash content of the fuel is high, the share of the returned mixture decreases.

If the fuel has a high content of volatile substances, there may be less product returned into the furnace. The amount of the returned mixture is regulated by the consumption regulator 13. Maintaining a high velocity of the bottom air flow to prevent drops and maintain large particles in the vortex zone is not a problem, and since the claimed method and the described furnace design provide that almost all the unburned particles are returned into the selected zone and then burned completely, the fuel combustion completeness is high.

The returned particles are a mixture of ash and unburned fuel (coke) particles, and fuel contains almost no volatile or water vapors, i.e. it comes to the selected fuel zone in the same way as if it was prepared thoroughly in pulverization devices. The claimed operation method of the swirling-type furnace and the swirling-type furnace therefore permit to burn crushed fuel with no pulverization devices required ensuring the production of steam with the required high parameters.

In the event sorbent must be used, the claimed furnace has one more benefit, as it is well known that by far not all of the sorbent that is supplied to the furnace chamber has time to react completely; sorbent particles that have not reacted are caught by the ash collector and returned to the furnace chamber with the flue dust. Sorbent is thus used many times.

INDUSTRIAL APPLICATION

The claimed technical invention enables a reconstruction of existing furnace units raising the ecological and economical parameters.

As experiments have shown, this design can work on various kinds of solid fuel including shale.

For tests on operating boilers with the claimed technical solutions, coal fuel was used with the following fractional composition: 5 to 15 percent sieve residue for 5 mm mesh and 50 to 70 percent residue for 1 mm mesh. The maximum particle size was 25 mm. The auxiliary power consumption (for draft, blast, and fuel feed) reduced from 9.18 kWh/ton of steam down to 7.9 kWh/ton of steam or 15 percent. 

1. An operation method of a swirling-type furnace containing a combustion chamber, having a vortex zone and an afterburning zone, a bottom air inlet device, and a coal and ash mixture catcher for flue gas including feed of the fuel/air mixture and bottom air to the furnace, characterized in that the fuel/air mixture containing coarse crushed fuel is supplied to the vortex zone of the chamber, and a controlled amount of the coal/ash mixture caught from the flue gas is returned to a selected zone in the inner space of the combustion chamber.
 2. An operation method of a swirling-type furnace as in claim 1, characterized in that the selected zone in the inner space of the combustion chamber is selected to be the vortex zone.
 3. An operation method of a swirling-type furnace as in claim 1, characterized in that the selected zone in the inner space of the combustion chamber is selected to be the afterburning zone.
 4. A swirling-type furnace containing a combustion chamber with a dry-bottom hopper formed by the slopes of the bottom parts of the combustion chamber walls, a bottom air inlet device under the dry-bottom hopper mouth, and a downward-tilted burner to supply the fuel/air mixture, which is installed on the combustion chamber wall; the furnace has an ash collector behind the combustion chamber, an ash circulation duct with a dust transportation device; one end of the said channel connects to the said ash collector, and the other end connects to the combustion chamber inner space; the outlet of the said channel is in the area of the selected zone of the inner space of the combustion chamber.
 5. A swirling-type furnace as in claim 4 characterized in that the coal/ash mixture consumption controller is installed in the ash duct.
 6. Swirling-type furnace as in claim 4 characterized in that there is a sorbent input medium in the ash duct.
 7. A swirling-type furnace as in claim 4 characterized in that the outlet of the circulation ash duct is placed between the dry-bottom hopper mouth and the fuel feed burner, and the ash collector is selected so as to catch particles that are larger than 0.5 mm.
 8. A swirling-type furnace as in claim 4 characterized in that an extra burner is installed on the combustion chamber wall in the afterburning zone, the outlet of the circulation ash duct is combined with the said burner, and the ash collector is selected so as to catch particles that are larger than 0.5 mm. 